DISTKIBUTION AND EVOLUTION 101 



such as the present. We will now proceed to a brief considera- 

 tion of the nature and meaning of the facts set forth in the 

 preceding chapters. 



The evidence, collected with extreme care for many years 

 by Mr. Woodruffe-Peacock (as explained in Chapter 11.)? has 

 shown us how curiously the number of species differs even 

 on the smallest adjacent areas. In the same field, even when 

 apparently alike everywhere in soil, in aspect, and in contour 

 of surface, every plot of 16 feet square has its individuality. 

 It will differ from each of the eight adjacent plots either in 

 tlie number of the species it contains, or in the species them- 

 selves, or in the proportions of the individuals of the various 

 species. They are thus seen to be affected by very small 

 differences, such as moisture, or aridity; more or less 

 shade from hedges, trees, or woods; shelter from or ex^^osure 

 to winds; by the vicinity of pits or quarries, woods, ponds, 

 or streams. 



Now this one fact of response to the minutest change of 

 conditions in the arrangement of a few^ species over almost 

 identical adjacent areas is as much a case of adaptation to 

 the environment through the mutual interaction of the various 

 species — a struggle for existence on the very smallest scale 

 ' — as any of those larger and more complex cases which Dar- 

 win first made known to us. 



Coming now to the fields themselves of various shapes and 

 dimensions, and each limited by definite boundaries of hedge 

 and ditch, bank or wall, spinneys, plantation or woods, we 

 have, in our country especially, a series of unit-areas which 

 may be said to form the first step in the study of botanical 

 geography, and which leads us on through successively larger 

 areas to regions and continents. 



In regard to these fields, the writer above quoted not only 

 states their precise differences in the numbers of their species 

 and the presence of certain species and absence of others which 

 give to each its individuality, but lie is able in many cases 

 to define the causes of that individiialitv. Besides the or- 



